An example of a technology for improving the transmission capacity of wavelength division multiplexing transmission systems is super channel transmission. In super channel transmission, a super channel is generated by virtually combining a plurality of sub-carriers (SCs). By performing, for example, Nyquist waveform shaping on each of the sub-carriers making up a super channel, the wavelength spacing between adjacent sub-carriers is decreased to a value close to the baud rate of a signal. Accordingly, by using super-channel transmission, the transmission capacity per bandwidth is increased. For example, if multilevel modulation, such as 16 quadrature amplitude modulation (QAM) is employed in transmission without a super channel, transmission of about 200 Gbps per bandwidth of 50 GHz is available. In contrast, in super-channel transmission, when two sub-carriers are within a bandwidth of 75 GHz, transmission of 400 Gbps per bandwidth of 75 GHz is available. That is, in the above-described example, the transmission capacity per bandwidth in the case of the super-channel transmission is increased to 1.3 times the transmission capacity in the case of non-super channel transmission. In super-channel transmission, the operations performed in an optical add/drop multiplexer (OADM) node, such as wavelength adding and dropping, are performed on a super-channel basis. At least two sub-carriers make up a super channel. Due to the restriction of the bandwidth of filters used in multiplexing and demultiplexing in OADM, the maximum number of combined sub-carriers is about 4.
The following optical transmission apparatuses are widely used. That is, optical transmission devices based on optical orthogonal frequency-division multiplexing (OFDM) divide an input payload into n pieces (n is an integer equal to 2 or greater), add forward error correction (FEC) codes having different error correction capabilities to the n divided pieces of the payload, and combine the n pieces of the payload having forward error correction codes added thereto. A payload combining unit places a payload having a higher error correction capability on a sub-carrier having a lower ratio of the sub-carrier power to the noise power (SNR) (refer to, for example, Japanese Laid-open Patent Publication No. 2013-192054).
The following error correction coding technique is also widely used. That is, input data is divided into predetermined length blocks, and error correction codes having different code lengths are provided to the blocks in accordance with the importance of the input data. Thus, error coding having error correction capability in accordance with the code length of error correction code is performed. In this manner, coded data having different packet lengths are generated in accordance with the code length of the error correction code (refer to, for example, Japanese Laid-open Patent Publication No. 11-127138).